To facilitate the wrapping of the cuff onto the finger, prior blood pressure monitors used on a finger employ the cuff wrapping mechanism shown in FIGS. 12 and 13. In FIG. 12, 122 is the case, and 123 is the cuff within the case. Rubber chamber 123b is attached to cuff 123.
The protruding end 123a of cuff 123 is fixed to lever 124. This lever is installed on case 122 in such a way that it can slide both in direction A, which tightens the cuff, and in direction B, which loosens the cuff. An elastic pawl 125 is provided on lever 124. As shown in FIG. 13, pawl 125 engages interdental spaces 126, which are located in two parallel rows along the path of movement of lever 124. Thus, the pawl prevents the lever from moving in direction B and loosening the cuff.
To measure a person's blood pressure, the person inserts a finger, usually the index finger, into opening 122a in case 122. Lever 124 is then moved in direction A to tighten the cuff. During tightening, pawl 125 is elastically deformed and passes over interdental spaces 126. When the appropriate cuff pressure has been attained, the operator releases lever 124, and pawl 125 engages in interdental spaces 126. This fixes the position of lever 124 and keeps cuff 123 wrapped around the patient's finger. Air is then pumped into rubber chamber 123b until the flow of blood in the finger artery is obstructed, and the blood pressure is measured.
When the blood pressure measurement has been completed, button 127 is pressed, causing pawl 125 to move down so that it is released from interdental spaces 126. Lever 124 slides in direction B, allowing cuff 123 to loosen.
With the existing cuff wrapping mechanism described above, the operator has a tendency to apply great force to lever 124 at the moment pawl 125 is slipping past teeth 126. This causes the pressure with which the cuff is wrapped to vary. For this reason, the tightness of the cuff is variable, and there is a pronounced tendency to overtighten it.
If cuff 123 is not wrapped sufficiently tightly, the volume of rubber chamber 123b will be increased, which will cause it to take longer for the cuff to be pressurized. Further, the area of the surface of rubber chamber 123b which contacts the finger will be decreased, which increases the chance that the rubber will wrinkle. If this happens, the blood pressure will not be measured correctly. Problems such as these are well known in blood pressure monitors employing the oscillation technique, in which the change in air pressure inside rubber chamber 123b is used to detect the pulse wave.
In contrast, if cuff 123 is wrapped too tightly, the blood in the finger can be completely obstructed, which will cause pain to the patient. To prevent cuff 123 from being wrapped too tightly, one could conceivably use deeper teeth for the toothed portions of 126, but this would cause the tightness to vary even more, and the operating noise would become much louder.